[llvm] [RISCV][TTI] Recognize CONCAT_VECTORS if a shufflevector mask is multiple insert subvector. (PR #111459)

[Luke Lau via llvm-commits]

================
@@ -343,6 +343,28 @@ RISCVTTIImpl::getConstantPoolLoadCost(Type *Ty,  TTI::TargetCostKind CostKind) {
                              /*AddressSpace=*/0, CostKind);
 }
 
+static bool isRepeatedConcatMaskImpl(ArrayRef<int> Mask, int &SubVectorSize) {
+  unsigned Size = Mask.size();
+  if (!isPowerOf2_32(Size))
+    return false;
+  for (unsigned I = 0; I != Size; ++I) {
+    if (static_cast<unsigned>(Mask[I]) == I)
+      continue;
+    if (Mask[I] != 0)
+      return false;
+    if (Size % I != 0)
+      return false;
+    for (unsigned J = 0; J != Size; ++J)
----------------
lukel97 wrote:

Nit, we can skip checking the first I elements
```suggestion
    for (unsigned J = I; J != Size; ++J)
```

https://github.com/llvm/llvm-project/pull/111459